Kumar IronUpdated
In this guide
What the coating layers are
A colour coated sheet is several layers on a steel base. JSW Coated Steel describes the typical build-up: a hot-dip galvanised (GI) or galvalume (AZ) steel coil as the base, a zinc or zinc-aluminium layer, a chemical pretreatment that bonds the paint to the metal, a primer that is oven-cured, a decorative topcoat, and a thin back coat on the reverse face.
Each layer has a job. The zinc stops the steel rusting from underneath. The pretreatment and primer make the paint stick. The topcoat gives the colour and resists the weather. The back coat protects the underside. If any layer is damaged, the one below it has to do the work, and it usually cannot do it for long.
Why the colour fades and chalks
Sunlight, especially UV, breaks down the paint over time. Two things usually happen. The colour pigment fades, and the binder in the paint slowly wears away so that a powdery residue, called chalking, comes off on your hand. Paint makers add UV blockers to slow this down, and the choice of topcoat matters a lot.
The main topcoats are polyester (PE), silicon-modified polyester (SMP), PVDF and high durability polyester (HDP). JSW describes polyester as the most widely used and cost-effective option for the Indian market, suited to inland sites where conditions are not harsh. SMP, where silicone is added to the resin, is harder and more resistant to chalking and fading. HDP is described as giving better colour retention over a longer period, and PVDF as keeping its colour and structure longest. Ask the dealer which topcoat is on the sheet you are buying.
| Topcoat | Common use | Stated strength |
|---|---|---|
| Polyester (PE) | Inland, budget projects | Most common and cost effective |
| Silicon modified polyester (SMP) | General roofing in mixed climates | Harder, resists chalking and fading |
| High durability polyester (HDP) | Longer colour retention needs | Better colour retention over time |
| PVDF | Harsh or coastal sites | Keeps colour and structure longest |
Scratches and cut edges
A scratch through the paint exposes the zinc and steel beneath. In a dry place the zinc protects the exposed area for a time, but in a damp or ammonia-laden shed the scratch can start to rust. Touch-up paint is a short-term fix. Keep scratches small, and report any large one to the dealer before the sheet goes on the roof.
Cut edges are the bigger issue. When a sheet is cut, the steel at the edge has no paint and often no full zinc layer, so it rusts first. Hot metal particles from a cutting disc or saw also land on the sheet and rust in small spots. Cut on the ground rather than on the roof, and brush off all swarf (metal shavings) from drilling and cutting. Everest's installation guide advises keeping edges clean and burr free to avoid rusting and injury.
- Cut sheets on the ground or on a clean surface, away from other sheets.
- Brush or vacuum off every metal chip from the sheet after drilling or cutting.
- Touch up exposed cut edges with paint only if the maker recommends it.
Cleaning and staining
Keep cement, mortar, lime, fertiliser and chemicals off the sheets. These can stain or attack the coating. If splashes do land, rinse them off with clean water soon after. Do not wait for them to dry and harden.
For routine cleaning, use clean water and a soft cloth or brush. Ask the sheet maker which cleaner they allow, since some harsh solvents or abrasives can damage the paint.
What to ask about the warranty
Every maker offers a warranty, but the terms differ. Do not judge a sheet only by the number on the advertisement. Ask for the written terms and read the conditions. Some makers cover fading, chalking and peeling, while others also cover rust. Check the period in writing, since the figure can change with the product and the region.
- Exactly what is covered: colour fade, chalking, peeling, rust, or only perforation.
- The warranty period in writing, and whether it applies from the date of purchase or installation.
- Conditions that can void it, such as poor storage, sheets left in contact with wet cement or lime, or unsuitable cleaners.
- Who handles the claim: the mill, the brand, or the dealer, and what proof you need.
- Whether the warranty covers the sheet only or also the labour to replace it.
Keep the bill, the batch or sheet markings, and photos of the storage and installation. These help if you have to claim.
Handling and storage on site
Most damage happens before the sheet reaches the roof. JSW warns that water, including condensation, trapped between sheets can damage the coating beyond repair, and it can cause white rust on the zinc. BlueScope's guidance is that bundles should be covered during transport and site storage, and that sheets should not touch wet lime, cement or soil.
- Store bundles off the ground, on bearers, so water cannot pool beneath them.
- Keep the bundle covered and ventilated, and do not let rain or dew collect between sheets.
- Keep the protective film on until the sheet is fixed, but remove it promptly. Sunlight can make the film stick to the paint, so do not leave it on for long in the sun.
- Carry sheets rather than dragging them. Use clean gloves so the surface is not marked by sweat or oil.
- Long sheets are safer with two people, carried on edge to avoid buckling.
- Walk on boards laid across the purlins, not on the sheets themselves.
If a bundle arrives wet, tell the dealer at once and ask how they want it handled before it goes on the roof.
Frequently asked questions
- Why did my colour coated sheet fade in two years?
- Usually the topcoat type, UV exposure, or a cheaper paint system. Ask the dealer which topcoat the sheet has, and keep the sheet clean. A written warranty and the batch details will help if you need to claim.
- Why does my sheet have white powder on the surface?
- That can be chalking, where the paint binder wears away, or white rust, which forms when water sits between sheets. Check which one it is by rubbing gently. Chalking comes off on the cloth, and white rust is a corrosion of the zinc.
- How do I stop rust starting at cut edges?
- Cut on the ground, clean off all swarf, and touch up cut edges with paint if the maker recommends it. Keep the sheet dry and avoid cutting over other sheets.
- Can I use a normal cleaner on a roof sheet?
- Use clean water and a soft brush. Ask the maker about any other cleaner first, because some chemicals can damage the paint.